In this slide discussed about the biological theories of the population by different famous biologist.

1. PRESENTATION ON-
BIOLOGICAL THEORIES ON POPULATION
SUBMITTED TO:- DR. A.K. BHARTIYA
(ASSOCIATE PROF. DSW)
SUBMITTED BY:- AVANISH KUMAR
MSW 2nd SEM.(REGULAR)
DEPARTMENT OF SOCIAL WORK
UNIVERSITY OF LUCKNOW

2. CONTENTS TABLE
Sr. No. Contents
1. Introduction
2. Sadler’s Density Theory
3. Thomas Doubleday’s Diet Theory
4. Pearl and Reed’s Logistic Curve Theory
5. Ginni’s Circular Theory
6. Herbert Spencer’s Theory
7. Castro’s Protein Consumption Theory
8. Conclusion

3. INTRODUCTION
Biological theories mainly based on biological aspects.
Biological theories mainly concerned with human fecundity.
Many of the social scientist proposed the different biological
theories.
Some important biological theories are as follows:-
{1} Density Theory
{2} Diet Theory
{3} Logistic Curve Theory
{4} Circular Theory
{5} Spencer’s Theory
{6} Protein Consumption Theory
The biological theorists believed in existence of metaphysical will
of nature to perpetuate the species.

4. Fecundity
Density
X
Y
P
P’
A
B
C
F1
F2
F3
O D1 D2 D3
Michael Thomas Sadler’s Theory /
The Density Principle
1780-1835
Work entitled as ‘The Law of Population’
in 1830
Define inverse relationship b/w The Density
of Population and The Fecundity
According to Sadler, such increase of
population and food supply in
mathematical terms just cannot happen,
because when population increases
density too will increase.
He believed that population adjusts itself
with the times.
People will be able to produce food
according to their needs and the food
supply and population will get adjusted
to each other.

5. It measures only in a given situation with the absence of the
technology
Population controlled with Happiness and Pleasure
Criticism of Sadler’s Theory
Sadler failed to distinguish between fecundity and fertility
Moreover, in many countries of the world where the density is high,
the fertility rate is also high. Even in India, in some states like Delhi
Sadler’s view that with industrialization the population decreases has
not been proved true. In a country like India, industrialisation has not
led to the decline in the growth of population.

6. Y
X
F1
F2
F3
Fecundity
Food Supply
S1 S2 S3
P
P’
Stage-ii
Stage-iii
Stage-i
THOMAS DOUBLEDAY’S DIET THEORY
Thomas Doubleday propounded the ‘true law of
population’
He stated the inverse relationship between food
supply and fecundity
Doubleday mentions three stages of food
supply, i.e.,
(i) Stage-I : The Plethoric state having
good food supply where the fecundity is
low,
(ii) Stage-II : The Deplethoric state
in which due to food shortage we find
diminution of proper nourishment
where the fecundity is high
(iii) Stage-III :When resources and fecundity
both are eqal.
O

7. d
Y
X
Q4
Q3
Q2
Q1
Q
P P1 P2 P3
size
Time
PEARL AND REED’S LOGISTIC THEORY
Theory titled in his book ‘The Biology of Population Growth’
This theory is also known as the ‘S’ shape theory
This theory is based on three assumptions:
(i) In the given time, measure the curve.
(ii) The number of population is
maximum.
(iii) The environment and population
are constant.
According to the graph 4 stages are
shorted out
(i) Rapid Growth: (Q-Q1)
(ii) Stable Condition: (Q1-Q2)
(iii) Decreasing Stage: (Q2-Q3)
(iv) Low Decreasing Stage: (Q3-Q4) O

8. Y
X
A
B
C
O
G3
G2
G1
S1 S2 S3
P
P’
F/PG
SD/ND
GINNI’S CIRCULAR THEORY
Ginni define the inverse relationship between the population growth
and social development and national development
This theory stated that Population growth impacts
upon the social and national development
Ginni was of the opinion that only biological
factors are responsible for the increase in
population and therefore his theory of
population can be characterised as a
‘natural law theory’
Ginni was of the opinion that due to high
fecundity population increases and
consequently social and economic
problems become complex

9. Circular Theory has four population
growth stages:
(i) Rapid Growth Stage
(ii) Slow Growth Stage
(iii) Maturity Stage
(iv) Decreasing Stage
Ginni believed that the population growth is similar to the cyclical
growth of an individual.
In the first stage, the growth of population is very rapid
while in the second stage, the growth is comparatively slow.
In the third stage which is known as senescence, population
decreases and there is deterioration in the quality of civilization
Maturity
Stable
Increasing
Slow Growth

10. Y
X
A
B
C
O
G3
G2
G1
S1 S2 S3
P
P’
Genesis
Individualization
HERBERT SPENCER’S THEORY
He stated that the population growth is
determined by ‘natural law’.
This theory is based upon the individualization
and genesis
He propounded the inverse relationship
between the individualization and genesis
Spencer shorted out three stages:
(i) Rich- Where individualization is
Increased and genesis is low i.e
The developed condition
(ii) Poor- Where individualization is low
and genesis is high i.e Underdeveloped
condition
(iii) Medium- Where individualization
And genesis both are equal i.e Developing
condition

11. CASTRO’S PROTIEN CONSUMPTION THEORY
He define his theory in his book
‘Geography of Hunger’ in 1952
He define the inverse relationship between the
protein consumption and the fecundity
According to Castro, deficiency of protein make
undernourished people more fertile than those who are well nourished
He conducted his experiment upon the rat and hen.
He categorize the rats in three groups and provide the protein
content in different ratio and shown the result :
Rats’ group Protein consumption % Reproduction capacity
(Births)
Group 1 10% 23.3
Group 2 18% 17.4
Group 3 22% 13.8

12. CONCLUSION
The Biological theories are mainly based on the Biological aspects
that are responsible for population growth.
These theories are mainly concerned with human fecundity.
The biological theorists believed in existence of metaphysical will
of nature to perpetuate the species.
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